I've been running betas/RC of both of these browsers over here and I've had very few issues. And one thing is for sure, if you're currently running Internet Explorer 7, then switching to either of these browsers will mean you gain speed, get improved CSS compliance, and consume less system memory. In fact, the truth is that you can't do any worse than run IE7. That browser really is a dog.
So, will one of these new versions entice you to change your default browser, or are you going to stay with what you know?
Me, I'm still on track to make the leap to Firefox 3.0.